Oxygen plasma treatment for producing high Tc oxide superconducting material
This is a continuation of application Ser. No. 195,145, filed May 18, 1988, now U.S. Pat. No. 4,900,716. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process for manufacturing a superconducting material, more particularly it relates to an aftertreatment of a sintered oxide type superconductor. 
     2. Description of the Related Art 
     The superconductivity is a phenomenon which is understood as a phenomenon of phase change of electrons under which the electrical resistance become zero and the perfect diamagnetism is observed. If the superconductor is applied to the present power cables which loses 7% of energy, electric current of a very high current density can be delivered without any loss of power. The superconductor can be utilized also on a form of a magnetic coil in the field of power electric applications such as MHD power generation, power transmission, electric power reservation or the like; in the field of transportation such as magnetic levitation trains or magnetically propelling ships; a high sensitive sensors or detectors for sensing very weak magnetic field, microwave, radiant ray or the like, in the medical field such as high-energy beam radiation unit; in the field of science such as NMR or high-energy physics; or in the field of fusion power generation. 
     In addition to the abovementioned power electric applications, the superconducting materials can be used in the field of electronics, for example, as a Josephson device which is an indispensable switching device for realizing a high-speed computer which consumes very reduced power. 
     However, the critical temperature could not exceed 23.2 K of Nb 3  Ge which was the highest Tc heretofore for all studies for the past ten years. 
     Possibility of existence of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Muller who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 189] 
     It had been known that certain ceramics material of compound oxides exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses Ba-Pb-Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba-Bi type compound oxides also show superconductivity. These type superconductors, however, possess a rather lower transition temperature of about 10 K and hence usage of liquidized helium (boiling point of 4.2 K) as cryogen is indispensable to realize superconductivity. 
     This new oxide type superconducting material discovered by Bednorz and Muller is [La, Ba] 2  CuO 4  which is called as the K 2  NiF 4  -type oxide having a crystal structure which is similar to known perovskite type oxide. The K 2  NiF 4  -type oxides show such higher Tc as 30 K which are extremely higher than the known superconducting materials. 
     It was also reported in the news parer that C. W. Chu et al discovered in the United States of America another type of superconducting material having the critical temperature of in the order of 90 K in February 1987, and hence possibility of existence of high-temperature superconductors have burst on the scene. 
     The abovementioned new type superconducting materials are usually produced in a form of bulky mass by sintering technique and in a form of a thin film by physical vapour deposition technique. In most case, the superconducting material produced by these techniques must be further subjected to thermal-treatment in an oxygen containing atmosphere in order to improve their property as superconductivity. 
     However, even after the thermal-treatment, the superconducting property of the oxide type superconductors is not stable and also the discrepancy (ΔT) between the onset temperature and the critical temperature can not be decreased. 
     Therefore, an object of the present invention is to overcome the abovementioned problems of the conventional technique and to provide a novel process for producing an improved superconducting material which has high Tc and high stability. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improvement in a process for producing a compound oxide type superconductor, characterized in that a compound oxide type superconducting material is treated in oxygen plasma. 
     The oxygen plasma can be produced by the conventional oxygen plasma reactor or generator, such as high-frequency plasma reactor. 
     The present inventors found such fact that a thin film deposited on a substrate by physical vapour deposition technique and then heat-treated in the oxygen containing atmosphere does not contain satisfactory oxygen atoms therein so that the film obtained can not possess a desired crystal structure of oxygen deficiency which is a critical factor for realizing the high Tc superconductivity. Therefore, it is necessary to introduce oxygen atoms into the thin film deposited. The present invention was completed from such finding. 
     The oxygen plasma is not thermally activated oxygen gas used in the conventional heat-treatment in oxygen gas atmosphere but have much activated or excited energy level so that oxygen atoms can penetrate into the crystal structure of the compound oxide, resulting in that the desired oxygen contents can be realized. 
     The treatment by oxygen plasma may be carried at any temperature, but it is preferable to heat the compound oxide to be treated at an elevated temperature. In fact, if the compound oxide is heated, the oxygen atoms doped into the compound oxide are diffused in the compound oxide to produce a homogeneous structure and to increase the oxygen contents. The compound oxide is preferably heated at a temperature ranging from 400° C. to 1,050° C. 
     The treated compound oxide is preferably cooled down slowly in the same atmosphere of oxygen plasma after the treatment by oxygen plasma complete at a cooling speed of no higher than 150° C./min. This slow cooling or annealing ensure that the doped oxygen is fixed in the crystal structure to realize a condition of oxygen rich. 
     The present invention is applicable to any compound oxide, particularly those that are difficult to realize the ideal crystal structure by the conventional technique and also is advantageously applicable to such compound oxides that can not exhibit satisfactory superconductivity due to imperfect condition during their manufacturing stage. Namely, the present invention permits to supplement them with additional oxygen so that they can be changed to satisfactory superconductors. 
     The compound oxide to which the present invention is applicable includes oxide type ceramics such as Y-Ba-Cu-O system, La-Ba-Cu-O system and La-Sr-Cu-O system. More generally, they belong to compound oxides including an element α selected from IIa group of the Periodic Table, an element β selected from IIIa group of the Periodic Table and copper (Cu). These constituent elements can be substituted partially by at least one of elements selected from a group comprising Al, Fe, Co, Ni, Zn, Ag and Ti. 
     Selection of an atomic ratio of α to β depend on combination of elements. For example, in cases of Ba-Y system, Ba-La system and Sr-La system, the following ratios are preferable: 
     Y/(Ba+Y) system: 0.06 to 0.94, preferably 0.1 to 0.4 
     Ba/(Ba+La) system: 0.04 to 0.96, preferably 0.08 to 0.45 
     Sr/(La+Sr) system: 0.03-0.95, preferably 0.05-0.1 
     When the atom ratios are not within the abovementioned range, high Tc can not be observed because the optimum conditions of crystal structure and oxygen deficiency can not be obtained. 
     The element α selected from IIa group of the Periodic Table is preferably Ba or Sr. The element β selected from IIIa group of the Periodic Table is preferably Y, La, Gd, Dy, Ho, Er, Tm, Yb, Nd, Sm or Eu or combinations of these elements. 
     The compound oxide can be produced by the conventional technique such as sintering or physical vapour deposition such as sputtering. When the compound oxide to be treated by the present invention takes a form of a bulky mass, it can be prepared by sintering directly or through a preliminarily sintering stage a powder mixture of oxide and/or carbonate of constituent elements of said compound oxide. If necessary, binder may be added to the powder mixture to produce a paste which is sintered later. 
     Selection of the sintering temperature and the preliminary sintering temperature depend on combinations of elements. In case of the abovementioned special combinations, they can be selected from the following rang: 
     
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system Preliminary sintering temp.                                        
                         Final sintering temp.                            
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Ba--Y  from 250 to 1,200° C.                                       
                         from 700 to 1,500° C.                     
Ba--La from 220 to 1,230° C.                                       
                         from 650 to 1,580° C.                     
Sr--La from 234 to 1,260° C.                                       
                         from 680 to 1,530° C.                     
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     The physical vapour deposition can be carried out by using the abovementioned compound oxide as a target. 
     The present invention is applicable to the compound oxide obtained by the abovementioned conventional technique. 
     The product treated by the process of the present invention from the abovementioned material of compound oxide is a compound oxide represented by the formula: 
     
         (α.sub.1-x β.sub.x)Cu.sub.y O.sub.z 
    
     in which α and β means the same definition as above, x is an atomic ratio of β withe respect to (α+β) having a range of 0.1≦x≦0.9, and y and z are respective atomic ratios with respect to (α 1-x  β x ) which is considered to be equal to 1 and each satisfy ranges of 0.4≦y≦3.0 and 1≦z≦5 respectively, and a compound oxide represented by the formula: 
     
         LnBa.sub.2 Cu.sub.3 O.sub.7-δ 
    
     in which Ln stands for at least one of elements selected from a group comprising Y, La, Gd, Dy, Ho, Er, Tm, Tb, Lu, Nd, Sm and Eu, and δ is a number which satisfies a range of 0&lt;δ&lt;1 and includes the following special systems: 
     
         ______________________________________                                    
Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,                                      
             Ho.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,                        
                           Lu.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,          
Sm.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,                                     
             Nd.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,                        
                           Gd.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,          
Eu.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,                                     
             Er.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,                        
                           Dy.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,          
Tm.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,                                     
             Yb.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,                        
                           La.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x,          
(La, Sr).sub.2 CuO.sub.4-x                                                
             (La, Ba).sub.2 CuO.sub.4-x                                   
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     in which x is a number which satisfies a range of 0&lt;x&lt;1. 
     The abovementioned oxides possess preferably perovskite type or quasi-perovskite type crystal structure. The term of quasi-perovskite type means a structure which can be considered to have such a crystal structure that is similar to Perovskite-type oxides and includes an orthorhombically distorted perovskite or a distorted oxygen-deficient perovskite or the like. 
     The present invention can produce further the other types of compound oxides including the following system: 
     
         Θ.sub.4 (Φ.sub.1-q, Ca.sub.q).sub.m Cu.sub.n O.sub.p+r 
    
     in which Θ stands for Bi or Tl, Φ stands for Sr when Θ is Bi and stands for Ba when Θ is Tl, m and n are numbers each satisfying ranges of 6≦m≦10 and 4≦n≦8 respectively, p=(12+2m+2n)/2, q is a number which satisfies a range of 0&lt;q&lt;1, and r is a number which satisfies a range of -2≦r≦+2. This system is considered to be a single phase of the following compound oxide or a mixed phase consisting mainly thereof: 
     Bi 4  Sr 4  Ca 4  Cu 6  O 20-r , Bi 2  Sr 2  Ca 2  Cu 3  O 10-r , 
     Tl 4  Ba 4  Ca 4  Cu 6  O 20-r , Tl 2  Ba 2  Ca 2  Cu 3  O 10-r , 
     in which r is a number which satisfies a range of -2≦r≦+2. 
     Now, the present invention will be described with reference to attached drawings which illustrate an apparatus used for carrying out the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is an illustrative view of a RF (radio-frequency) type plasma reactor which can be used in the present invention. 
    
    
     The apparatus shown in FIG. 1 includes a chamber 1, a high-frequency coil 2 mounted on the chamber 1 and a heater 4 for heating a material oxide 3 placed in the chamber 1. The chamber 1 is connected to a vacuum pump (not shown) through a port 7 for creating vacuum in the chamber 1. Oxygen gas is introduced through a port 6. 
     In operation, a compound oxide 3 is placed in the chamber and vacuum is created in the chamber 1. Then, oxygen gas is fed through the port 6 and the high-frequency coil 2 is energized. The heater 4 is actuated when the compound oxide required to be treated in a heated condition. 
     Now, the process according to the present invention will be described with reference to illustrative Examples which are carried out in the apparatus shown in FIG. 1, but the scope of the present invention should not be limited thereto. 
     EXAMPLE 1 
     In this example, a sintered powder block was used as a compound oxide. 
     At first, powders of Y 2  O 3  and BaCO 3  were mixed at an atom ratio of Y:Ba=1:2 and then a powder of CuO was added to the resulting powder mixture at an atom ratio of Y:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered preliminarily at 880° C. and then further sintered finally at 940° C. to obtain a sintered block of YBa 2  Cu 3  O 7  having a dimension of 20×30×3 mm. 
     After the block was placed in the chamber, the pressure in the chamber 1 was reduced to 10 -6  Torr, oxygen gas of 0.1 Torr was introduced in the chamber. Then, the high-frequency coil is energized at a power of 150 W and the block was heated gradually at a rate of 15° C./min to a temperature of 900° C. by the heater. After the block was left under this temperature for 3 hours, the block was cooled slowly at a cooling rate of 8° C./min (Sample A). 
     For comparison, the same procedure as above was repeated except that the high-frequency coil is not energized but the block was treated only in an oxygen atmosphere 760 Torr (Sample B). 
     Electrodes are vacuum-deposited on opposite ends of the block treated to measure resistance of the resulting block. Tc (critical temperature) and Tcf (a temperature where complete zero resistance is realized) measured on the resulting treated block are shown in Table 1. 
     EXAMPLE 2 
     The same procedure as Example 1 was repeated except that powders of Bi 2  O 3 , CaO, SrCO 3  and CuO were mixed at an atom ratio of Bi:Ca:Sr:Cu=2:2:2:3 and then sintered at 880° C. to obtain a sintered block of Bi 4  Sr 4  Ca 4  Cu 6  O 20 . 
     Oxygen plasma treatment was carried out on this block in the same manner as Example 1 (Sample A). 
     For comparison, the same procedure as above was repeated except that the high-frequency coil is not energized but the block was treated only in an oxygen atmosphere 760 Torr (Sample B). 
     The conditions of treatment and the result of Tc and Tcf measured is shown in Table 1. 
     EXAMPLE 3 
     As material powders, powders of La 2  O 3  and BaCO 3  were mixed at an atom ratio of La:Ba=95:5 and then a powder of CuO was added to the resulting powder mixture at an atom ratio of La:Ba:Cu=95:5:200. Then, the resulting powder mixture was sintered preliminarily at 870° C. and then further sintered finally at 955° C. to obtain a sintered block of (La,Ba)Cu 2  O 3   
     The same oxygen plasma treatment as Example 1 was repeated (Sample A). 
     For comparison, the same procedure as above was repeated except that the high-frequency coil is not energized but the block was treated only in an oxygen atmosphere 760 Torr (Sample B). 
     The conditions of treatment and the result of Tc and Tcf measured is shown in Table 1. 
     EXAMPLE 4 
     A material powder of BaCO 3  in Example 3 is displaced by SrCO 3  and the same procedure as Example 3 was repeated to obtain a sintered block of (La,Sr)Cu 2  O 3   
     The same oxygen plasma treatment as Example 1 was repeated (Sample A). 
     For comparison, the same procedure as above was repeated except that the high-frequency coil is not energized but the block was treated only in an oxygen atmosphere 760 Torr (Sample B). 
     The conditions of treatment and the result of Tc and Tcf measured is shown in Table 1. 
     EXAMPLE 5 
     A thin film of the compound oxide obtained in Example 1 was deposited on a substrate made of SrTiO 3  in an ordinary sputtering machine by using the bock prepared in Example 1 as a target. 
     The oxygen plasma treatment according to the present invention (Sample A) and the conventional heat-treatment in oxygen gas of 760 Torr (Sample B) were repeated in the same manner as Example 1 
     The conditions of treatment and the result of Tc and Tcf measured is shown in Table 1. 
     EXAMPLE 6 
     A block of sintered powder was produced by the same procedure as Example 1 except that the powder mixture was sintered simply once at 840° C. The block did not show any superconducting property 
     Then, the block was treated in the apparatus shown in FIG. 1 with oxygen plasma under an oxygen pressure of 10 -1  Torr at a high-frequency power of 150 W. The other condition is shown in FIG. 1 with the result of measurement of Tc and Tcf. 
     This example shows that a compound oxide which do not possess superconducting property can be changed to a proper superconductor by the present invention. 
     
                                           TABLE 1                                 
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Condition of oxygen plasma treatment                                      
    O.sub.2                                                               
         RF  Substrate                                                    
                   Heating                                                
                        Cooling                                           
Ex. pressure                                                              
         power                                                            
             temp. rate rate  Tc Tcf                                      
No. (Torr)                                                                
         (W) (°C.)                                                 
                   (°C./min)                                       
                        (°C./min)                                  
                              (K)                                         
                                 (K)                                      
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1 A 1 × 10.sup.-1                                                   
         150 900   15   8     113                                         
                                 105                                      
 B  760  --  900   15   8     82 51                                       
2 A 1    100 400   10   10    98 95                                       
 B  760  --  400   10   10    80 40                                       
3 A 2 × 10.sup.-1                                                   
         150 1,000 15   150   98 90                                       
 B  760  --  1,000 15   150   76 39                                       
4 A 1 × 10.sup.-1                                                   
         150 1,050 15   8     106                                         
                                 99                                       
 B  760  --  1,050 15   8     63 24                                       
5 A 7 × 10.sup.-2                                                   
         150 800   15   8     101                                         
                                 92                                       
 B  760  --  800   15   8     65 12                                       
6   1 × 10.sup.-1                                                   
         150 900   15   8     91 82                                       
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